There has been an increasing demand for remotely controllable circuit breaker assemblies that can reciprocate between an open circuit and a closed circuit in response to a remotely generated command. One advantageous application for such circuit breaker assemblies is in control panelboards that are used for automated control systems such as automated lighting systems. Automated lighting systems have been developed for the control of lighting circuits based upon inputs such as the time-of-day, wall switches, occupancy sensors and/or control from a power distribution system. Lighting control systems offer an opportunity to save energy by automating the process of cutting back on the number of lighting fixtures that are illuminated, or by cutting out artificial lighting altogether when circumstances warrant. For example, ambient light sensors can be used to control lighting circuits in response to ambient light levels. The sensors can serve both switching and automatic dimming functions that can adjust the output of the lighting system continually in response to the amount of daylight striking the ambient light sensor. Occupancy sensors can be used to activate lighting when someone is in a space and to deactivate the lighting, perhaps after a set time interval, when a person is no longer detected in the space.
In general, circuit breaker assemblies that can be remotely controlled may be divided into at least two classes. The first is the remote-operated circuit breaker. In a remote-operated circuit breaker, two pairs of contacts are located within a single package. The first (or primary) pair of contacts is used to interrupt short circuits, to interrupt overloads, and to switch the circuit breaker on and off via a handle. The second pair of contacts in a remote operated circuit breaker may be used, for example, in a lighting control application. Those secondary contacts are intended to be switched more often than the primary pair of contacts, but do not have the robustness to maintain their intended function if exposed to the arc and heat associated with a short circuit. It is therefore important that the secondary pair of contacts be maintained in a closed position when “large” currents (for this purpose 1,000-20,000 amperes) are passed through the remote operated circuit breaker. Without the incorporation of specific design features, electromagnetic forces tend to open those secondary contacts under large current loads before the primary contacts interrupt the circuit, causing arcing and heating and potentially damaging the contacts.
Another class of remotely controlled circuit breaker assemblies is an assembly that includes a circuit control pod. In such an assembly, a relay device or “pod” (with means to operate a pair of contacts remotely) is attached to a standard circuit breaker that does not have a means of remote operation. The circuit control pod adds an additional pair of contacts in series with the circuit breaker. Like the secondary contacts of the remote-operated circuit breaker described above, the secondary contacts of the circuit control pod must be held closed during short circuit and overload conditions. If the secondary contacts are not held closed, the interruption of a short circuit may be split between the circuit breaker and the circuit control pod. Under those conditions, there is a high risk that the circuit control pod would be damaged.
Several designs have been proposed for preventing contacts from blowing open under increased current loads. For example, it is known to use a spring to maintain electrical contacts in a closed position.
U.S. Pat. No. 5,301,083 discloses a contact pair having a moveable contact arm with a hold-down electromagnet that exerts increasing force with increasing current through the contact arm.
U.S. Pat. No. 6,034,581 discloses a contact assembly in which parallel current flow in the moveable contact arm and adjacent conductors creates attractive and repulsive forces that hold the contacts together to resist unintended separation.
There is presently a need for an improved design and method for keeping a pair of contacts closed during a short circuit. Such a design should have a low cost and should be of high reliability. Such a design should furthermore be compact for use in a small package area. Accordingly, it is an object of this invention to provide a reliable, low cost and compact remotely controllable circuit breaker assembly. To the inventors' knowledge, no such remotely controllable circuit breaker assembly is currently available.